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  #1  
Old 10-13-2003, 09:33 PM
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Bizzarro OVP problem solved!

Just wanted to post the ending of my hard start problem for anyone else that might run into the same that I did. Just to review, the car was very hard to start when cold, fine when warm. When I checked the fuse on the OVP, it was blown. After I had replaced it with a good fuse, the car would not start at all.
As Gilly had guessed, someone had probably tried to fix the hard start problem by fiddling with the mixture adjustment, but to no avail because the fuse was blown and the car was running in closed loop. I adjusted the mixture screw under the airfilter cover, first one way and then the other until I found which way made a difference. It was a good 3 complete turns out I think from where it should have been at, but now it runs perfectly and was very easy to do. Actually the biggest pain was trying to get the bypass and vacuum hose connected back on the engine while at the same time connecting it to the bottom of the air filter cover. I found that it was actually easier to connect the hose on the airfilter cover first, and then connect the others on the engine, as I had a difficult time trying to push the breather hose onto it's much larger mount for some reason. Thanks for all the help.

Steve
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  #2  
Old 10-13-2003, 09:47 PM
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Steve:
Glad you got it running better. Let me explain just a few things though:
You stated the car was in "closed loop". This is incorrect. If anything you should consider it open loop. When it is "open loop" it means the car hasn't warmed up enough for the O2 sensor to properly work, and the engine is operating with the mixture set almost exclusively by the engine coolant temp sensor. Once the engine warms up sufficiently, the mixture is being set by the O2 sensor, this is known as "Lambda". What that means is that when the O2 sensor senses "rich mixture", the control module drives the mixture to the lean side. When the O2 sensor then senses "lean mixture", the control unit then enriches the mixture. This is Lambda, once the engine warms up the mixture is set using this Lambda procedure, contantly switching between rich and lean. Being in Lambda operation is also called "closed loop", the relationship between the O2 sensor and control unit is "closed". If the control unit isn't operating in Lambda, because the engine temp isn't warm enough, it is "open" loop, no relation yet between the O2 sensor and control unit.

However what was going on with your car was different. The control unit was essentially "dead" because of the blown fuse in the OVP, so the system has sort of an emergency running mode, which is called "fixed operating mode", or FOM for short. This is different than being in open loop. It's not even taking the engine coolant temp into account, the mixture is a constant, which is why it would start OK warm, but not very well cold, the mixture is set to run best at normal operating temp, that's how it's set up to run in FOM, at normal operating temp, no variation.

Gilly
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  #3  
Old 10-13-2003, 11:15 PM
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Did I say closed loop? Of course, I meant open loop. And yes, I do understand the FOM mode, but thought that the 02 sensor still would be sending a signal to the ECU. I guess that without the fuse, the control unit is as you stated, "dead" and no sensor information is being used to determine the air/fuel mixtures. But while in the FOM mode would making mixture adjustments then have any effect on the running of the car? The adjustments that I made were mostly mechanical, not electrical from what I can tell, as it looks like it just mechanically adjusts the air sensor plate height to determine the air/fuel mixture. So I am thinking that it should still effect the running of the engine to some degree even while in FOM.

Steve
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  #4  
Old 10-13-2003, 11:29 PM
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No, there is no power running to the air flow sensor when the control unit isn't powered up. The control unit has to provide power to the air flow sensor for it to operate, so it's dead.
To illustrate this, go out to the car now with it working right, with it running and press down on the air flow sensor plate. The car will die. Then do the same with the fuse pulled out of the OVP. It won't change it at all.
When you adjust the mixture screw, you are making a mechanical adjustment to an electrical part (the air flow sensor). If there is no electricity making it through the sensor you aren't doing anything but screwing it up for whoever is the poor sucker who gets the power eventually flowing through it. Sound familiar?

Gilly

PS It's kind of like the old trick of unscrewing the cap on the salt shaker as you leave the resturaunt
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  #5  
Old 10-14-2003, 11:42 AM
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Gilly, a followup question. I had my duty cycle on when I depressed the air flow plate. The reading increased before the car died. Could you please explain why the car dies & why the reading increases? Thanks.
Richard
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  #6  
Old 10-14-2003, 07:55 PM
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Normally air flow is what causes the air flow sensor plate to go down. The mixture is enrichened due to the increased airflow. If you depress the air flow sensor plate, the engine doesn't "know" it was your finger that did it, it assumes that the airflow actually did increase, the amount dependent on how far you pushed the plate. It incorrectly enriches the mixture, enriches it enough to stall the engine. The duty cycle changing is a reflection on the enrichment that is being done by the EHA (Electro Hydraulic Actuator).

Gilly
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  #7  
Old 10-14-2003, 08:01 PM
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So, would the stall suggest that the EHA is being properly powered by the OVP, & that the ECU is getting correct information?
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  #8  
Old 10-14-2003, 08:04 PM
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Mmmmmmmm, it would tell you the control unit is working and the air flow sensor is probably "OK", if not perfect. I wouldn't say it's an indication that the on-off ratio (mixture) is set correctly. What did you set the on-off ratio to? Was the adjustment tower "uncorked" on top? You're supposed to replace the adjustment tower if the ball is gone.

Gilly
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  #9  
Old 10-14-2003, 10:35 PM
Bud
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What would cause the fuse to blow on the OVP. Isn't this an indication of another problem?

TIA,
Bud
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  #10  
Old 10-14-2003, 10:38 PM
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The OVP itself is bad, or there was a surge or incorrect polarity when jump starting the car, usually somehting along those lines.

Gilly
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  #11  
Old 10-15-2003, 10:55 AM
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Gilly,
I've been unable to set the mixture because regardless of my adjustments to the screw (I haven't done more than a total of 1/2 a turn at any one session), the duty cycle reading stays fixed at 90%. I should add that before I bought the car the OVP fuse was blown, & I had it replaced. Can I safely try to attempt resetting to a 45% mixture by major screw movement/enrichment, which I believe involves turning the screw clockwise? BTW, the tower is open on top, & I use a 3mm allen wrench for adjustments. Thanks.
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  #12  
Old 10-15-2003, 12:13 PM
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Gilly,

Through this whole thread no one has said what year and model were are discussing, but the EHA gives me some clues. I never heard the term" fixed operating mode. I'm guessing it would run rough cold because there would be no reduced control pressure like the basic KE which had a warmup compensator and also didn't use a temp sensor. It would be operating just like the KE system and have to be running lean when cold with the electronics dead. Now I'm curious enough to pull the EHA plug on one of my cars and see how it runs cold.

Richard28,
That constant 90% you are seeing is not a duty cycle. It is either saying you have a car with onboard testing system or you have a fault code stored and you will have to look it up in the code book. I don't have my books at home to look it up for you. Sorry.

Peter
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  #13  
Old 10-15-2003, 08:41 PM
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You can unplug the entire control unit and the engine will STILL run. This is called "Fixed Operating Mode".

Peter is right, this is the very early version of a car having a diagnostic capability, although still no memory. I don't know what a fixed 90% is either, I'll see if it's on my 124 disc and report back later.

Gilly
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  #14  
Old 10-16-2003, 10:18 AM
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A 90 reading is the dreaded "Current to electrohydraulic actuator implausible." This is accompanied by the following footnote:

"The plausibility of the road speed signal can only be checked by the KE control unit when driving. If an implausible road signal is recognized when driving, the control unit "sets" the on/off ratio of 60% and stores this. The fault is not erased unit the ingnition is switched off. A "60% on/off ratio" test is performed, e.g. in dealing with the complaint [of] jerking when vehicle moving and throttle valve closed."

I got this reading once on my late 300E. I concluded that it had something to do with the road speed sensor. But then it did not reappear so I never pursued it further. I have always been curious about what it really means.
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  #15  
Old 10-16-2003, 09:27 PM
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The info I have on my 201 disc is that 90% is not an assigned code. There is an 80%, and 100%, but no 90%.

Gilly
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